vhost-vsock: fix migration issue when seqpacket is supported

vhost-vsock: fix migration issue when seqpacket is supported

Commit 1e08fd0a46 ("vhost-vsock: SOCK_SEQPACKET feature bit support")
enabled the SEQPACKET feature bit.
This commit is released with QEMU 6.1, so if we try to migrate a VM where
the host kernel supports SEQPACKET but machine type version is less than
6.1, we get the following errors:

    Features 0x130000002 unsupported. Allowed features: 0x179000000
    Failed to load virtio-vhost_vsock:virtio
    error while loading state for instance 0x0 of device '0000:00:05.0/virtio-vhost_vsock'
    load of migration failed: Operation not permitted

Let's disable the feature bit for machine types < 6.1.
We add a new OnOffAuto property for this, called `seqpacket`.
When it is `auto` (default), QEMU behaves as before, trying to enable the
feature, when it is `on` QEMU will fail if the backend (vhost-vsock
kernel module) doesn't support it.

Fixes: 1e08fd0a46 ("vhost-vsock: SOCK_SEQPACKET feature bit support")
Cc: [email protected]
Reported-by: Jiang Wang <[email protected]>
Signed-off-by: Stefano Garzarella <[email protected]>
Message-Id: <20210921161642[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>